It has been known for many years that the flow rate of a fluid, e.g. hot water flowing along a pipe, can be measured by measuring the respective propagation times of ultrasonic signals emitted upstream and downstream between two ultrasonic transducers situated at points spaced apart in the flow direction of the fluid.
In the field of measuring the flow rate of hot water, Document WO 86/02722 discloses a method consisting in causing both transducers to emit respective ultrasonic signals simultaneously, the two signals thus propagating in opposite directions.
Because of the presence of the flow, the propagation time T2 of the signal emitted upstream is longer than the propagation time T1 of the signal emitted downstream.
By measuring the two propagation times T1, T2, it is possible to deduce the hot water flow rate therefrom by using the formula Q=K(T2-T1)/C, where K is a term taking into account the geometry of the meter, and C is a correction term related to the speed of propagation of sound in water.
That method suffers from a major drawback. When one of the transducers has just been excited, it continues to emit a signal while it is receiving the signal coming from the other transducer. When the temperature of the water varies, drift has been observed, and additional unwanted phase shifts are observed in the received ultrasonic signals.
To overcome that problem, it is necessary to take temperature measurements and to correct the flow rate measurements as a function of fluctuations in temperature, which complicates the measurement method.
In addition, other measurement methods are known such as, for example, the method described in Document EP 0 426 309, in which method acoustic signals, each of which includes a phase inversion, are emitted consecutively in mutually opposite directions into a flowing fluid. The propagation time of each the received acoustic signals is measured by detecting the instant at which the phase inversion appears relative to a time reference which is related to the emission signal in question.
That instant is detected by means of an instantaneous phase detector, but such detection is not accurate.
For each of the acoustic signals, the time measurement is associated with a measurement of the acoustic phase shift induced in the acoustic signal in question because of the signal propagating in the flow.
The acoustic phase shift is measured by sampling the received signal on eight capacitors, by digitizing said sampled signal, and by performing synchronous detection on the resulting digitized signal.
Unfortunately, because of the sampling, that measurement method imparts additional noise to the sampled values of the signal, and thus to the measurement itself.
In addition, that method is complex because it requires the propagation time and the acoustic phase shift to be measured for each signal launch in a given propagation direction.
Therefore, it would be advantageous to find a measurement method that does not impart additional noise to the measurement, and that is simpler to implement than in the prior art.